Field of the Invention
The present invention relates to an image pickup apparatus including an image pickup device, a method of determining a light amount change characteristic at the time of photographing, and a storage medium.
Description of the Related Art
In recent years, image pickup apparatuses, such as a digital camera, have come to be equipped with an image pickup device improved in sensitivity. This has made it possible to photograph a picture without blurring at a high shutter speed even under a relatively dark environment, such as indoors. However, fluorescent lamps, which are widely used as indoor light sources, produce a phenomenon called flicker in which illumination light periodically fluctuates due to influence of the frequency of a power supply (in general, a commercial power supply). When image pickup is performed at high shutter speed under such a light source that produces flicker (hereinafter referred to as the “flickering light source”), exposure unevenness or color temperature variation of an image can be caused between frames due to the influence of flickering, and further, exposure unevenness and/or color unevenness sometimes occur(s) within one frame.
To solve this problem, there has been proposed a technique for detecting a state of flicker of illumination light, and adjusting the timing of image pickup such that the center of an exposure time substantially coincides with timing in which the light amount of the illumination light indicates a maximum value. For example, in a technique described in Japanese Patent Laid-Open Publication No. 2012-120132, to detect flicker and its frequency, first, a plurality of images are acquired (photographed) at a high-speed frame rate not lower than 600 fps, and a luminance evaluation value is obtained from each frame image. Then, temporal changes in the luminance evaluation value are subjected to binarization processing, and if an interval of the timing in which the luminance evaluation value after binarization processing rises from 0 to 1 is 10 ms (milliseconds), it is determined that 100 Hz flicker is caused, and if the interval is 8.4 ms, it is determined that 120 Hz flicker is caused.
The technique disclosed in Japanese Patent Laid-Open Publication No. 2012-120132 is useful in a case where the light amount of flicker ideally changes along a sine curve. However, in a case where the light amount of flicker irregularly changes e.g. due to the characteristics of a power source or a lighting (illumination) device, there is a possibility that it is impossible to correctly determine occurrence of flicker and its frequency.